I use Mobil I fully synthetic Extended Performance oil in our Class A with an 8.1L chevy engine. I have made trips from GA to NV without changing the oil with no problems.
This is from the Mobil 1 website:
To prove the protection of our oils in some of the toughest conditions, we test in real situations. Watch the testing of Mobil 1™ Extended Performance in Las Vegas taxis, where engines are pushed to the limit. The oil was changed every 15,000 miles.